Despite substantial progress in the past two decades, cancer remains the leading cause of death by disease in US children between 1 and 15 years of age. Acute lymphoblastic leukemia (ALL) is the most common childhood cancer, and cure rates are approaching approximately 80% today. Unfortunately, 20% of children with ALL are not cured with current therapy, making the number of cases of relapsed ALL greater than the total number of new cases of most childhood cancers. Previous work has established that de novo drug resistance is a primary cause of treatment failure in childhood ALL. However, the genomic determinants of such resistance remain poorly defined. We have recently identified a number of new genes that are expressed at a significantly different level in B-lineage ALL cells exhibiting de novo resistance to widely used antileukemic agents (prednisolone, vincristine, asparaginase, daunorubicin), and their pattern of expression was also significantly related to treatment outcome. To assess, three research aims that extend our prior findings. The first scientific aim is to identify genes conferring de novo resistance of childhood ALL to the widely used thiopurines, mercaptopurine and thioguanine. This will be the first genome-wide analysis of genes conferring thiopurine resistance and will provide important new insights into whether they represent distinct antileukemic agents. The second aim is to identify genes in T-ALL that confer de novo resistance to the four agents we have previously studied in B-lineage ALL (prednisolone, vincristine, asparaginase, daunorubicin) and the two thiopurines. This will yield pharmacogenomic insights into why T-ALL has a worse prognosis with most treatment protocols. The final aim is to identify germline polymorphisms or epigenetic changes in the promoter regions of those genes that are differentially expressed in ALL cells exhibiting resistance to these antileukemic agents. Preliminary studies have already identified a significant relation between mRNA expression in ALL cells and the promoter haplotype structure of the first gene investigated (SMARCB1). It is important to extend these pharmacogenomic studies in a systematic way to additional genes conferring de novo drug resistance. These findings will continue to provide important new insights into the genomic determinants of treatment failure and point to novel targets for developing strategies to overcome drug resistance in childhood ALL.